Patent Document 1 discloses an impurity particles measuring device that includes a light emitting diode emitting light to a pipe through which oil (hydraulic oil) or the like flows, two light receiving elements arranged apart from each other in a direction of the flow path of a fluid and detecting light transmitted through a flow path by the light emission, and a detecting section detecting an amount of impurity particles flowing in the flow path from differences in respective outputs of the light receiving elements, and that measures impurity particles, such as oil dust contained in the hydraulic oil and the like.